The present invention relates to a base station apparatus and radio communication method in a digital radio communication system.
A conventional radio communication system is explained. FIG. 1 is a schematic view showing a conventional radio communication system using sector antennas. FIG. 2 is a schematic view showing a conventional radio communication system using variable directivities.
First, the conventional radio communication system using sector antennas is explained with reference to FIG. 1. For example, base station 1 carries out communications with three fixed directivities toward sector antenna (fixed directivity) A, sector antenna (fixed directivity) B and sector antenna (fixed directivity) C. Normally, setting such fixed directivities in different directions is called xe2x80x9csectorization.xe2x80x9d Base station 1 communicates with terminal A2 with fixed directivity A. Base station 1 further communicates with terminal B3 with fixed directivity B.
Thus, a radio communication system using many fixed directivities improves its reception characteristic using a diversity technology for increasing the number of antennas on the base station side. In a mobile communication environment in particular, the diversity technology compensates a drop of reception field intensity due to fading.
Next, a conventional radio communication system using variable directivities is explained with reference to FIG. 2. For example, base station 1 communicates with terminal A2 with variable directivity A. Base station 1 further communicates with terminal B3 with variable directivity B.
Thus, carrying out communications with a narrow directivity formed for every terminal makes it possible to improve frequency utilization. This technique is reported in TECHNICAL REPORT OF IEICE Axc2x7P96-131, etc. In addition, a system of performing control for every terminal so that a desired signal is received in optimal conditions and transmitting using a weighting factor generated at that time is described in ICIEC.Trans.COMMUN., VOL.E77-B, No.5 MAY, 1994.
Moreover, a method for performing control for every terminal so that a desired signal is received in optimal conditions, using the improved reception quality for a reduction of transmit power on the terminal side and increasing the subscriber capacity of the uplink is described in TECHNICAL REPORT OF IEICE IT96-66 (1997-03). The concept of transmit power control is reported in TECHNICAL REPORT OF IEICE Axc2x796-155 (1997-02).
However, according to the conventional system, if the size of sectors is reduced, handover takes place more frequently between different sectors, which complicates communication control by a base station interrupting communications. Furthermore, in a CDMA communication system, identical signals spread by different spreading codes, are sent from a plurality of sectors to a same terminal to prevent the communication from being interrupted during handover. This function is called xe2x80x9cdiversity handover.xe2x80x9d However, signals directed to a same terminal are sent to a plurality of sectors during diversity handover, causing a shortage of the communication capacity on the downlink (communication channel from a base station to a terminal).
Moreover, in the technique disclosed in ICIEC.Trans.COMMUN., VOL.E77-B, No.5 MAY, 1994, reception signal correlation is about 1 because a signal is received by an array antenna. In a mobile communication environment, this will result in considerable deterioration in the characteristic when the intensity of a reception signal drops due to fading. Therefore, the communication quality needs to be compensated by diversity as in the case where sector antennas are used. However, since array antennas to form variable directivities are large and expensive, it is difficult to perform diversity with a plurality of array antennas.
Moreover, in the technique disclosed in TECHNICAL REPORT OF IEICE IT96-66 (1997-03), a drop of field intensity due to fading in a mobile communication environment is compensated by transmit power control on the terminal side. However, this needs high-speed transmit power control to follow up the mobile communication environment, requiring an expensive transmission amplifier for the terminal. Furthermore, since deterioration of the reception quality at the base station is compensated by the terminal, transmit power of the terminal increases, whereas the communication time and wait time are shortened.
Furthermore, since the conventional base station apparatus does not have a configuration that allows the number of terminals accommodated by the sector antennas or the number of terminals accommodated with variable directivities to be increased as required, the base station needs to be replaced.
It is an object of the present invention to provide a base station apparatus and radio communication method, which are easy to control, resistant to fading in a mobile communication environment and capable of changing the number of terminals accommodated with variable directivities.
A main subject of the present invention is to increase the subscriber capacity in the system, taking account of a good reception characteristic of a communication apparatus with variable directivities, by having the communication apparatus with variable directivities positively accommodate communications with terminals in poor reception conditions and reducing power of these terminals to reduce interference.